Summerlyn doesn't have sidewalks, so the children would either walk in the yards or streets (which is not a good thing) Summerlyn doesn't have tennis, and the pool isn't built yet. The pool is being built by the developer, not Lennar who is building the houses. If you inquire about the pool, they will tell you it is "planned". You have to be very careful if a pool is important to you since it isn't built yet. It is possible they will cancel the pool, as this has happened in the past.
Both are good communities and have their pros and cons. Cobblestone is located closer to the bypass, so commutes to Raleigh are a bit shorter. One thing I would suggest is look at the area around those neighborhoods. The area around Cobblestone has smaller and older homes, while the area around Summerlyn has larger and higher priced homes. Also, Summerlyn is across from the new Clayton Town Park, which will be a good place for the children to play when it is finished, but you'll have to drive them over when it's open, you wouldn't want them crossing Glen Laurel Rd to get there, way too much traffic on that road for children to cross alone.
Summerlyn will be in the new Powhatan Elem that is opening this fall, so no track record is available for it right now. It will be getting a lot of children from East Clayton, and the principal from East Clayton will be the new principal at Powhatan, so I think looking at East Clayton will give you a feel for how Powhatan will perform. Cooper did fail to meet the requirements of No Child Left Behind last year, and is facing sanctions for that.
Let me know if you have anymore questions for the Clayton area, it's a great place to live, and I love it here.
They both have similar amenities ( pool, tennis, clubhouse). You have to decide which is most suitable for your family's needs. Sometimes it comes down to personal preference.
The schools are definitely very important and you should take that into account for resale value as well.
Here is the link to the schools:
I recommend you visit the schools and get your own opinion from the staff. That may help you decide.